Seat back



FIG. 1 is a perspective view of a seat back showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a side elevational view, the opposite side being a mirror image thereof;

FIG. 4 is a rear elevational view thereof; and,

FIG. 5 is a top view thereof.

The views of the seat back not shown form no part of the claimed design. The broken lines depicting a seat portion represent environmental elements only and form no part of the claimed design. 

The ornamental design for a seat back, as shown and described. 